Imaging devices having extended dynamic ranges have been known heretofore. For example, Japanese Patent Application Laid-Open No. 10-261077 (pages 3-5 and FIG. 1) discloses an imaging device including imaging means that has a horizontal CCD transfer rate twice as high as that of an ordinary transfer rate and is capable of outputting a long-exposure-time video signal and a short-exposure-time video signal in one field period.
In conventional imaging devices, imaging means performs pixel addition. A video signal output from the imaging means is subjected to preprocessing such as CDS (Correlated Double Sampling) and AGC (Automatic Gain Control) in preprocessing means and then is converted to a digital signal by A/D converting means. The digital signal is then input in time axis conversion means and separated into a long-exposure-time image signal (LONG signal) and a short-exposure-time image signal (SHORT signal) which are transmitted at a standard rate and the same timing.
Level mixing means receives the LONG and SHORT signals, compresses the LONG signal which has reached the saturation point and adds an offset to the SHORT signal. The level mixing means generates a mixed signal so as to obtain a LONG signal when the luminance level is lower than a mixing point and obtain a SHORT signal when the luminance level is higher than the mixing point, where the mixing point (mixing level) is the saturation point.
Histogram data detecting means detects a histogram of the luminance of the mixed signal. Block data detecting means divides the mixed signal into blocks and detects the average luminance of each block. Based on the detected average luminance, area border position data calculating means determines area border positions. Grayscale correction data calculating means calculates a grayscale correction characteristic in each of the areas into which the screen is divided. Area grayscale correction means corrects the grayscale on the basis of the grayscale correction characteristic.
In accordance with the calculated area border positions, area division pulse generating means generates a control pulse for dividing, and for combining, the area of the mixed signal. Area combining means combines images the grayscales of which have been corrected on the basis of the control pulse. Main signal processing means performs main signal processing of the imaging device and outputs a signal. In this way, the conventional imaging device mixes LONG and SHORT signals to provide an image having an extended dynamic range.
However, in conventional imaging devices, pixels of four colors, Mg (magenta), G (green), Cy (cyan), and Ye (yellow), are arranged in a predetermined pattern on an imaging area of an imaging element. For example, pixels are arranged in a conventional imaging element in such a manner that a pixel line of Mg and G (first pixel line) and a pixel line of Cy and Ye (second pixel line) are alternately formed. Because pixel addition is performed in the conventional imaging devices in order to improve the sensitivity of the imaging element, the resolution of the imaging element decreases accordingly.
It could be contemplated that an all-pixel readout imaging element which does not perform pixel addition is used in order to provide images with a higher resolution. However, pixels of the four colors of the imaging element have different sensitivities to incident light and therefore different pixel lines of the imaging element have different sensitivities to incident light. Accordingly, if an all-pixel readout imaging element is simply used in a conventional imaging device, the first and second pixel lines can exhibit different luminance levels (signal levels) even in an area in which the pixels should have the same luminance level due to different sensitivities of the pixels because pixel addition is not performed. For example, when the quantity of light having a wavelength to which Cy and Ye pixels are highly sensitive is small in a bright area in which a SHORT signal should be used, second pixel lines (pixel lines of Cy and Ye) will be interpreted as a dark area and a LONG signal will be used in the area.
If the conventional level mixing described above is simply performed, SHORT signal pixel lines (for example the first pixel lines) and pixel lines changed to LONG signal lines (for example the second pixel lines) exist in an area in which the same exposure time signal should be used. As a result, line-to-line luminance differences are produced in an image obtained by mixing LONG and SHORT signals.